Let's say you've freed up a couple days and more than a couple bucks to visit San Francisco. Unlike the hordes of tourists who visit this city each year, you'd rather not spend your entire visit doing predictable activities like slurping on crab legs at Fisherman's Wharf and photographing the heck out of the Golden Gate Bridge. You're all for urban culture, good eats and fancy shoes, and maybe even an evening at the theater, but your deeper allegiance lies with ferns and boulders and watersheds, the lineaments of the land and the elemental realities of place.

So explore the city's trees.

You can enjoy this kind of special tourism in many cities, but San Francisco might be the best place for it. The trees here are unlike other cities' trees, and they're certainly not like the native communities found in wilder locales: the scruffy manzanita-oak association, the tidy ponderosa pine old growth. Here in the city where towering glass walls rise from endless rolling concrete-clad hills, you can have a unique experience enjoying the disturbed.

Most people don't realize that historically the San Francisco Peninsula was largely treeless, with only a few live oaks and California buckeyes huddled in the occasional sheltered valley. Now, roughly 230 years after settlement by Europeans and other invaders, the peninsula is home to one of the densest cities in the U.S., and within that city, a reported 274 different tree species. That's an astonishing number, considering the challenges of the habitat: sandy and severely compacted soils, fierce winds and fogs, dry summers, pavement, the occasional wayward taxicab or dump truck that gouges a trunk or snaps a branch.

Trees here also have to deal with the whims of homeowners, each with his or her own slice of sidewalk to "manage." In 1981 a municipal budget cut eliminated tree-planting programs in most neighborhoods, so now, roughly 85 percent of street trees are cared for by residents. "I don't like this tree because it drops gunk on my car," someone might say, or, "These branches are blocking my second-story window." Sometimes, in the fashion of a Ferrari-buying midlife crisis, people just get bored with the trees they planted years ago and replace them with something different.

For these reasons and others, it's no exaggeration to say that the trees that have "made it" in San Francisco are, by necessity, a unique bunch. Outside of Monterey cypresses, Monterey pines, and coastal redwoods -- all of which, due to their size, are more often found in parks than neighborhoods -- very few of the city's trees are native to California. Roughly 40 percent hail from New Zealand and Australia, and many others are from the Mediterranean. On a single block in San Francisco you can see Brazilians, Chinese, Guatemalans, Irish and Malaysians, all of them standing in a row. Trees, that is, not people.

How can you engage these patient, fragrant, exotic, textured, resilient shade-makers? Is it enough to simply keep your eyes peeled while wandering from café to café? Would you know a Canary Island palm if you saw one? Could you differentiate it from a windmill palm, a Mexican fan palm or a cabbage palm (the last of which is actually more closely related to yuccas and agaves)? And more to the point: what about finding the best Canary Island palm in the entire city?

Recommendation: The Trees of San Francisco by Mike Sullivan, a guidebook-cum-field guide soon to be released in an expanded second edition, currently available at 24 branch libraries across the city as well as in local bookstores and online. Other West Coast cities boast similar titles -- Exceptional Trees of Los Angeles, Trees of Greater Portland, Trees of Seattle -- but those guidebooks can be a bit heavy on the botany and light on adventure. The Trees of San Francisco works like a treasure map. Like the other books, it provides descriptions of species and street addresses for exemplary "landmark trees." Unlike the others, it links these landmark trees, along with sites of architectural, historical and general scenic interest, into walking tours.

Sullivan, the author, is a 53-year-old corporate lawyer who wears suits, works in a skyscraper, and describes himself as "sort of an idiot-savant when it comes to San Francisco street trees." Mention any block in the city and he'll likely know what's growing there, and not just the common names but the Latin binomials as well. In reference to some obscure corner he'll say, "Oh, there's a 10-car pileup of amazing species there!"

On weekends, when he's not volunteering with Friends of the Urban Forest (he's planted trees with the group for nearly three decades, and for three years was Chair of the Board), you might find Sullivan walking one of his own tours with a clutch of would-be-urban-botanists in tow. There's no need to hook up with the man himself, though. His expertise, his quirky stories about the hows and whys behind the trees, and his passion for the city he calls home -- it's all in the book. Strolling with The Trees of San Francisco is like strolling with Sullivan in your backpack, only lighter.